Conventionally, heat-shrinkable tubes composed of mainly polyvinyl chloride have been used widely as electrical insulation materials for use in applications such as covering capacitors. In recent years, for making lighter, thinner, shorter, and smaller products, electronic components such as capacitors have been densely assembled, and fields such as electric components for automobiles whose operating temperature is high have been expanding rapidly. For these reasons, heat-shrinkable tubes have been required to have favorable heat resistance. The heat-shrinkable tubes made of polyvinyl chloride have problems such as insufficient heat resistance, and generation of hydrogen chloride gas during combustion, which is likely to damage an incinerator in the case of disposal such as incineration. For this reason, as alternatives to the heat-shrinkable tubes composed of polyvinyl chloride, heat-shrinkable tubes composed of crystalline resins such as polyester-based resins or polyphenylene sulfide-based resins have come to be used.
Above all, polyphenylene sulfide-based resins are excellent in not only heat resistance, but also flame retardancy, chemical resistance, electrolytic solution resistance and the like, and thus preferable for electric components of automobiles and electronic component applications. Attention has been focused on the characteristics to consider heat-shrinkable tubes using polyphenylene sulfide-based resins (Patent Document 1). However, the tube obtained in accordance with the method described in Patent Document 1 has a problem that the high glass transition temperature Tg of the resin fails to achieve shrinkage for a short period of time. The rate of step of covering a capacitor or the like with the tube is increased under recent production conditions, in other words, for the purpose of improvement in productivity, thereby resulting in a failure to apply the tube under production conditions of which covering conditions are at a higher temperature and a shorter period of time.
In order to solve the problems described above, the inventors have proposed a tube which is composed mainly of a polyphenylene sulfide-based resin and excellent in low-temperature shrinkage characteristics. However, when the tube is exposed to high temperatures insteps such as assembling a capacitor, problems, for example, relaxation of the tube once closely attached to a component such as a capacitor due to crystal growth (heat-resistant swelling) are pointed out. Thus, further improvements to the heat-resistant swelling have been required in applications such as covering capacitors. Patent Document 1: Japanese Patent Application Laid-Open No. 9-157402